1. Field of the Invention
The invention concerns methods, apparatus, and systems for handoff in a wireless communication system.
2. Related Art
A subscriber station in a wireless communication system is very often in motion. The motion of the subscriber station introduces variations into a communication channel between the subscriber station and the base station. For example, the motion of the subscriber station introduces a Doppler shift into the communication channel. The subscriber station and the base station can attempt to compensate for the effects of motion of the subscriber station. In this regard, typical wireless communication systems can adjust the transmit frequency in very fine increments to pre-compensate for the effects of Doppler shift in the channel caused by movement of the subscriber station.
A typical communications system can be configured to operate a communications channel with one or more subscriber stations over some predetermined range of operating conditions. The operating conditions can include, for example, conditions for signal power, temperature and signal modulation type.
Typically, a receiver in a communications system is configured to support operations over some predetermined worst case conditions. For example, the receiver can be configured to properly operate without compensation over an entire range of an operating condition, or it can be configured to correct or otherwise compensate for one or more effects of an operating condition when that operating condition is outside of certain range values. For example, the receiver may include a filter whose bandwidth varies based on the temperature. In another example, the receiver can be configured to implement a filter having a fixed bandwidth that is selected to provide sufficient performance over the entire temperature range, or the receiver can be configured to monitor a temperature and to compensate the filter for the effects of temperature.
In the case that the subscriber station is moving, it may be desirable to modify the transmit frequency of the base station and/or the subscriber station based on an estimate of the movement rate of the subscriber station. However, typical methods for estimating the movement rate of the subscriber station in known communication systems are cumbersome and inefficient. In addition, it may be desirable switch an operating parameter of the subscriber station which is related to alternative services based on whether the subscriber station is moving.